This invention relates to the processing of tip treatment bars of a rotor casing for a gas turbine engine.
WO94/20759 discloses an anti-stall tip treatment means in a gas turbine engine, in which an annular cavity is provided adjacent the blade tips of a compressor rotor. The cavity communicates with the gas flow path through the compressor past a series of bars extending across the mouth of the cavity.
Such tip treatments are applicable to both fans and compressors of gas turbine engines, and their purpose is to improve the blade stall characteristics or surge characteristics of the compressor.
The passage of the blade tips past the bars creates vibrations in the bars which, over time, can result in fatigue failure of the bars.
It is an object of the present invention to increase the fatigue life of the bars.
It is a further object of the present invention to avoid premature failure of the bars, and consequent damage to the engine.
According to one aspect of the present invention, there is provided a method of processing tip treatment bars of a rotor casing for a gas turbine engine, the method comprising subjecting the bars to a process which induces compressive stress within the bars.
According to another aspect of the present invention there is provided a gas turbine engine having a bladed rotor which is rotatable within a casing, the casing being provided with an array of spaced tip treatment bars disposed adjacent the blade tips of the rotor, in which the bars have been subjected to a process which induces compressive stress within the bars.
The process applied to the bars is preferably a surface process, and may be a peening process. A shot peening process may be used, but in a preferred application of the invention, a laser shock peening process is used.
The process is preferably applied to at least one side of each bar (i.e. a face having a radial component of direction with respect to the engine axis). Preferably, both sides of-the bar are subjected to the process.